The present invention relates to an optical bistable device on a single semiconductor chip.
FIG. 1 shows a typical construction of a conventional optical bistable device, and FIG. 2 shows its equivalent circuit. This type of device is integrated and formed with a photodiode PD of double heteroconstruction and a laser diode LD placed face-to-face on a semiconductor substrate 1 separated by a groove 2. The layers of the photodiode PD and laser diode LD are composed, for instance, as follows:
1 - N+type GaAs substrate PA1 3 - N+type GaAlAs layer PA1 4 - P+type GaAs layer PA1 5 - P+type GaAlAs layer PA1 6 - P+type GaAs layer.
A common cathode electrode 7 is formed on the rear side of substrate 1 and anode electrodes 8 and 9 on the uppermost layers, respectively, of the photodiode PD and the laser diode LD. Output light is emitted from the laser diode LD at both end sides and part of the output light received by photodiode PD passing through groove 2.
In the above construction, the positive pole of a suitable DC power source is connected to the anode electrode 9 of laser diode LD, while the negative pole is connected to the electrode 8 of photodiode PD. Simultaneously, a suitable bias circuit is connected to the common electrode 7. Therefore, if an input light signal Pi is received by photodiode PD, a current reverse flows in photodiode PD and this current becomes the driving current in a forward direction for the laser diode to generate an output light Po from laser diode LD. This ouptut light is received by photodiode PD to further increase the reverse current and further intensifies the output light Po by positive feedback. Therefore, the output light Po is maintained at a stable level almost to a fixed value.
FIGS. 3 and 4 illustrate the input and output characteristics of the light emitting and receiving elements of the laser diode LD and photodiode PD. FIG. 3 is a curve showing the relation of input current to output light intensity of the laser diode LD. The output light intensity of the laser diode LD rapidly increases if the input current value exceeds a certain threshold value A. FIG. 4 is a curve showing the relation of input light intensity versus output current of the photodiode PD. The output current of the photodiode PD almost proportionately increases until reaching a value B. Once the value B is exceeded, the current stably flows at a fixed level C. FIG. 5 shows the relation between the input and output of the light emitting laser diode LD and the light receiving photodiode PD. The equivalent circuit for the arrangement of FIG. 1 is shown in FIG. 6.
In FIG. 6 both diodes PD and LD are arranged so that the output light Po of the laser diode LD is incident upon photodiode PD. The voltage of a DC power source E is suitably divided by resistors R1, R2, and R3. Bias voltages are applied, respectively, to the photodiode PD in a reverse direction and to the laser diode LD in a forward direction.
Receipt of input light signal Pi by photodiode PD produces a reverse direction current flow in photodiode PD and this current is an input to the laser diode LD. When the input light Pi reaches a certain change value X1, the output light Po rapidly increases toward a high level as shown in FIG. 5. Output current of the photodiode PD flows stably at value C when the intensity of the input light exceeds a certain value B. Accordingly, the output light Po of the laser diode LD is emitted at a certain stable level Y.
If thereafter the input light Pi decreases, the output current of the photodiode PD similarly decreases. However, the output light Po of the laser diode Ld being partially fed back to the photodiode PD, the output current decreases at a smaller gradient as shown in FIG. 5. When the input light signal falls below change value X2, the intensity of the output light Po of the laser diode LD changes rapidly from a high to a low level. Value X2 is lower than value X1. Therefore, the input and output characteristics display hysteresis, and, the input light Pi can be formed as a bianry code at a certain threshold value for waveform shaping of an optical output signal.
To change the values X1 and X2 of such a conventional optical bistable device, the values of resistors R1, R2 and R3 must be changed. Consequently, once the resistances have been chosen and the resistors assembled in a circuit, change or adjustment of the values X1 and X2 is extremely difficult.
The present invention relates to an improved bistable optical device in which a light modulating element is mounted on or integrated into the semiconductor chip and is responsive to a control signal to vary the light from the photo emitting element received by the photo responsive element and thereby vary as desired the values X1 and X2. An electrochemical crystal can be used for the light modulating element.
Other objects and purposes of the invention will be clear from the following detailed description of the invention.